Various heat transfer recording methods have been known so far. Among these methods, dye diffusion transfer recording systems attract attention as a process that can produce a color hard copy having an image quality closest to that of silver halide photography. Moreover, this system has advantages over silver halide photography: it enables direct visualization from digital data; it makes reproduction simple, and the like without using treatment chemicals.
Among these methods, in a sublimation type thermal transfer recording system, a heat-sensitive transfer sheet (hereinafter also referred to as an ink sheet) containing dyes is superposed on a heat-sensitive transfer image-receiving sheet (hereinafter also referred to as an image-receiving sheet), and then the ink sheet is heated, for example, by a thermal head whose exothermic action is controlled by electric signals, in order to transfer the dyes contained in the ink sheet to the image-receiving sheet, thereby recording an image information. Three colors: cyan, magenta, and yellow, are used for recording a color image by overlapping one color to other, thereby enabling transferring and recording a color image having continuous gradation for color densities.
In recent years, because speeding up of a printer can shorten user's waiting time in case that print is conducted in a photo shop for user's advantage, high-speed printers in the sublimation-type thermal transfer recording system, which can provide a print in a short time, have been developed and commercialized one after another.
In wide spread use of the printers in the sublimation-type thermal transfer recording system, there is a demand for a printer providing prints of good image quality under various environmental conditions, independently of an installation site. That is, in order to satisfy user's needs, it is necessary to provide a print of good image quality and free from image defect, not only under the standard air-conditioned environmental condition of offices and shops, for example, at a temperature of 23° C. to 27° C. and a humidity of 50% to 70%, but also, under a high-temperature high-humidity condition in summer (e.g., temperature: 35° C., humidity: 80%) and a low-temperature low-humidity condition in winter (e.g., temperature: 10° C., humidity: 20%).
As one of the technical problems to be solved in the sublimation-type thermal transfer recording system, there is pointed out an image trouble that is caused by a release failure between the heat-sensitive transfer sheet and the heat-sensitive transfer image receiving sheet. In the sublimation-type thermal transfer recording system, the heat-sensitive transfer sheet and the heat-sensitive transfer image receiving sheet are superimposed with each other to form an image. The image is formed on the heat-sensitive transfer image receiving sheet. Therefore, after image formation, it is necessary to strip off the unneeded heat-sensitive transfer sheet from the heat-sensitive transfer image receiving sheet without leaving any unwanted substances on the image receiving sheet. However, from a requirement of reduction in printing time, there is a trend that a temperature applied to the heat-sensitive transfer sheet at the time of recording is raised (a heating time is reduced in keeping with this trend). Consequently, this trend is easy to cause a problem that, after printing, the print and the heat-sensitive transfer sheet are fused to each other, or the heat-sensitive transfer sheet is not successively separated from the heat-sensitive transfer image receiving sheet and thereby a releasing line remains.
For prevention of the heat-sensitive transfer sheet and the heat-sensitive transfer image receiving sheet from fusing, several methods of using a releasing agent such as a silicon compound or a fluorine compound have been proposed. As one of these methods, a method of introducing such the releasing agent into the image receiving sheet has been proposed. However, in a recent sublimation-type thermal transfer recording system, a transparent resin is laminated, in many cases, on the heat-sensitive transfer image receiving sheet having an image formed thereon from viewpoints of improvement in both scratch resistance and light fastness of the formed image. In this embodiment, when a releasing agent is present in the side of the heat-sensitive transfer image receiving sheet, the releasing agent gives, in some cases, disadvantages to the transparent resin laminate.
As an alternative, a method of introducing the releasing agent into the heat-sensitive transfer sheet has been proposed. As the releasing agent that is introduced in this method, a usage of fluorine-system polymers has been also proposed (see Japanese Patent No. 3150691, JP-A-5-32072 (“JP-A” means unexamined published Japanese patent application) and JP-A-1-146791).